Download Preparing projected entangled pair states on a quantum computer

Preparing Projected Entangled Pair States on a
Quantum Computer
Martin Schwarz, Kristan Temme, Frank Verstraete
University of Vienna, Faculty of Physics, Boltzmanngasse 5, 1090 Vienna, Austria
Toby Cubitt, David Peréz-García
Mathematics and Quantum Information group, Departamento de Analisis Matematico,
Facultad de CC Matematicas, Universidad Complutense de Madrid,
Plaza de Ciencias 3, Ciudad Universitaria, 28040 Madrid, Spain
Phy. Rev. Lett. 108, 110502 (2012), arXiv:1104.1410
Overview
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Some PEPS background
Growing PEPS
Run-time bound
Generalizations
Conclusion
PEPS properties
• Projected entangled-pari states (PEPS) are a natural
parameterization of multi-partite quantum states defined on lattices
• Conjectured structure of ground states of gapped local Hamiltonians
• Proven so by Hastings[1] for 1D case (Matrix Product States)
• General PEPS preparation oracles are powerful resources![2]
– as powerful as contracting tensor networks or computing their norm
– PP-complete, for general PEPS given as classical input
Key question raised by Verstraete, Wolf, Peréz-García, Cirac (2006)[3]:
Is it possible to prepare PEPS in BQP under mild restrictions?
If PEPS generation is hard, what subclass of PEPS are physical?
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[1] M. Hastings, Journal of Stat. Mech. 2007, P08024 (2007)
[2] N. Schuch, M. Wolf, FV, and J.I. Cirac, PRL 98, 140506 (2007)
[3] FV, M. Wolf, DPG, and J. Cirac, PRL 96, 220601 (2006)
PEPS definition
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PEPS are quantum states defined on
an arbitrary graph G=(V,E).
Edges of G are associated to
maximally entangled pairs states
providing virtual indices.
A tensor A(v) of rank k+1 is associated
to each vertex v of degree k mapping
k virtual indices to one physical index.
PEPS
is created by applying the
maps A(v) to the virtual indices.
Injective PEPS
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A PEPS is called injective, iff the map A(v) is left-invertible (after blocking
sites), i.e.
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Injectivity is a generic property of many interesting states
(e.g. 2D AKLT, all lattices, etc. almost always injective)
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Each injective PEPS is the unique ground
state of a parent Hamiltonian[4]
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There is a standard construction[4] for
2-local parent Hamiltonian
H = projector onto the complement
of the support of each 2-body reduced
density matrix of PEPS
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[4] DPG, FV, J. I. Cirac, and M. M. Wolf, Quant. Inf. Comp 8, 0650 (2008)
Growing PEPS
Key idea for preparing injective PEPS:
1. prepare a set of entangled pair states, one for each edge
2. construct parent Hamiltonian with entangled pairs as groundstate
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treat “virtual“ PEPS indices as physical indices as well
3. grow the PEPS by growing the parent Hamiltonian, vertex by vertex
1. update parent Hamiltonian edge and vertex terms
2. add Hamiltonian term to restrict to the physical subspace with energy
penalty larger than spectral gap D
1. project onto unique ground state vector of updated Hamiltonian
– This step is probabilistic and requires some more work…
4. final ground state is the PEPS we want to prepare
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Growing PEPS
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Growing PEPS
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Growing PEPS
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Growing PEPS
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Growing PEPS
…
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Projecting on next ground state
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Projection onto the next ground state is performed using Phase Estimation[5]
We perform a binary measurement on the energy register (zero or non-zero)
If the outcome is zero, we have perpared the desired ground state
Else, we undo the measurement using the well-known Marriot-Watrous trick[6]
and re-try, either starting from the original state or an orthogonal one
By Jordan´s Lemma, analysis reduces to a single 2x2 block, i.e. 2D subspace
Transition may also be effected adiabatically[7] (doesn‘t generalize to G-inj. case)
unique
GS of Ht+1
unique GS of Ht
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[5] M. Nielsen, I. Chuang. Quant. Comp. and Quant. Inf., Cambridge Univ. Press. (2000)
[6] C. Marriot, J. Watrous, Comput. Complex. 14, pp. 122-152. (2003)
[7] D. Aharonov, A. Ta-Shma, SIAM J. Comput. Vol. 37, No. 1, pp. 47-82, (2007)
Bounding the run-time
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We need a lower bound for the transition probability
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Let
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Then, using the PEPS structure, we show that for each injective PEPS
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Let k=max(k(A(v))) be the max. condition number over all PEPS projectors A(v)
Let D=min(D(Ht)) be the smallest spectral gap over all Ht generated
Then the algorithm produces PEPS p with probability at least 1-e in time
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be the condition number of matrix A
Generalization
• The algorithm generalizes to G-injective PEPS[8], where symmetry
group G is acting on virtual indices and A(v) is left-invertible on the Ginvariant subspace. A PEPS is called G-isometric, if all A(v)´s are
isometries.
• Problem: A(v)´s not injective
 parent Hamiltonian has degenerate ground space!
 How can we undo failed projections which are not rank-1?
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[8] N. Schuch, J.I. Cirac, DPG, Annals of Physics, Volume 325, Issue 10 (2010)
Growing G-injective PEPS
• Generalization of the basic algorithm
– A related G-isometric PEPS is prepared deterministically by known
methods[9,10] to enter the G-invariant subspace first
– The G-isometric PEPS is then transformed into the G-injective PEPS as
before, maintaining the G-invariant subspace
– To undo measurements, we crucially use the PEPS structure and the fact
that A(v) is invertible on the the G-invariant subspace to show that in fact
we can proceed as before!
• Same run-time bound as basic algorithm.
• Examples of G-isometric PEPS being prepared initially:
– G=Z1:
– G=Z2:
(trivial):
product of entangled pairs (same as before)
well-known toric code state
• The class of G-injective PEPS includes many physically interesting
states with topologically order, such as quantum double models, etc.
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[9] M. Aguado, G. Vidal, Phys. Rev. Lett. 100, 070404 (2008)
[10] FV, M. M. Wolf, and J. I. Cirac. Nature Physics, 5:633–636 (2009)
Conclusion
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We have shown how to efficiently (in BQP) prepare well-conditioned
injective PEPS on a quantum computer with high probability.
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We have exploited the PEPS structure to construct a sequence of parent
Hamiltonians with an induced sequence of unique ground states with lowerbounded overlap, and we have shown how to move through this sequence
efficiently to produce the final PEPS.
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The result generalizes to G-injective PEPS yielding the same run-time bound
of (with k restricted to the G-invariant subspace)
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Future directions:
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– implement quadratic speedup in k by using Quantum Rejection Sampling[11]
– can we solve interesting computational problems in this PEPS framework faster?[12]
[11] M. Ozols, M. Roetteler, J. Roland, arXiv:1103.2774 (2011)
[12] I. Arad, Z. Landau, SIAM Journal on Computing 39, 3089 (2010)
References
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M. Hastings, Journal of Statistical Mechanics: Theory and Experiment 2007, P08024 (2007)
N. Schuch, M. Wolf, F. Verstraete, and J.I. Cirac, PRL 98, 140506 (2007)
F. Verstraete, M. Wolf, D. Perez-Garcia, and J. Cirac, PRL 96, 220601 (2006)
D. Perez-Garcia, F. Verstraete, J. I. Cirac, and M. M. Wolf, Quant. Inf. Comp 8, 0650 (2008)
D. Aharonov, A. Ta-Shma, SIAM J. Comput. Vol. 37, No. 1, pp. 47-82, (2007)
C. Marriot, J. Watrous, Comput. Complex. 14, pp. 122-152. (2003)
M. A. Nielsen, I. L. Chuang. Quantum Comp. and Quantum Inf., Cambridge Univ. Press. (2000)
N. Schuch, J.I. Cirac, D. Peréz-García, Annals of Physics, Volume 325, Issue 10 (2010)
M. Aguado, G. Vidal, Phys. Rev. Lett. 100, 070404 (2008)
F. Verstraete, M. M. Wolf, and J. I. Cirac. Nature Physics, 5:633–636 (2009)
M. Ozols, M. Roetteler, J. Roland, Quantum rejection sampling, arXiv:1103.2774 (2011)
I. Arad, Z. Landau, SIAM Journal on Computing 39, 3089 (2010)